> All what I need is selector for ID, html function, ajax call, > element toggling and sliding (slideDown).
The first three are easy enough that you don't need a library, you could just use trivial wrappers around getElementById, innerHTML, and XMLHTTPRequest. If you are willing to settle for simple immediate show/ hide, you could again get by without a library and just use a simple change to the CSS display attribute. All the code for that would probably be 2KB max. If you really need animated toggling and sliding, that is tricky to get right on all browsers and I would just use the unmodified of jQuery; it isn't really that big considering what it offers you. Once it's down on the client it is cached.
